Resinous compositions curable through a transesterification curing mechanism

ABSTRACT

Coating compositions comprising a polymeric polyol with a polyester crosslinking agent having at least two beta- and/or gamma-ester ester groups per molecule are disclosed. The compositions, when applied to a substrate and cured in the presence of a transesterification catalyst, give solvent-resistant coatings.

CROSS-REFERENCE TO COPENDING APPLICATIONS

Application Ser. No. 288,238, filed even date herewith discloses coatingcompositions which are heat-curable to give solvent-resistant coatingscomprising a polymeric polyol, a polyester crosslinking agent having atleast two beta-alkoxyester groups per molecule and a transesterificationcatalyst.

Application Ser. No. 288,239, filed even date herewith discloses coatingcompositions which are heat-curable to give solvent-resistant coatingscomprising a polymeric polyol, a crosslinking agent having at least twogamma- and/or delta-hydroxyester groups per molecule and atransesterification catalyst.

Application Ser. No. 288,241, filed even date herewith discloses coatingcompositions which are heat-curable to give solvent-resistant coatingscomprising a polymeric polyol, a crosslinking agent having at least twobeta-amido ester groups per molecule and a transesterification catalyst.

BACKGROUND OF THE INVENTION

Field of the Invention: The present invention relates to heat-curableresinous coating compositions and to the use of these coatingcompositions in cationic electrodeposition. More particularly, thepresent invention relates to resinous coating compositions which curethrough a transesterification reaction.

Brief Description of the Prior Art: U.S. Pat. No. 3,937,679 disclosescationic heat-curable resinous coating compositions such as hydroxylgroup-containing polymers in combination with aminoplast resin curingagents. These compositions can be used in an electrodeposition processwhere they coat out on the cathode, and when cured, produce coatingswith excellent properties. Coating compositions using aminoplast curebest in an acidic environment. However, the deposit on the cathode isbasic and high curing temperatures must be used to overcome theunfavorable curing environment.

U.S. Pat. No. 4,101,486 is similar to U.S. Pat. No. 3,937,679 in that itdiscloses cationic electrodeposition of hydroxyl group-containingpolymers, however, the curing agent is a blocked isocyanate. Coatingcompositions using blocked isocyanates cure very well at relatively lowtemperatures in a basic environment and are today widely used inindustrial cationic electrodeposition. Examples of cationicelectrodepositable compositions which are used industrially are thosedescribed in U.S. Pat. No. 4,031,050 and 4,190,564 and DE-OS 2,752,255.Although used extensively throughout the electrodeposition industry,blocked isocyanate-containing compositions are undesirable from thepoint of view of the isocyanate, some of which are undesirable tohandle.

European Patent application 0012463 discloses thermosetting resinouscoating compositions which cure through a transesterification reaction.The resinous binder of the coating composition comprises ahydroxyl-containing polymer and a crosslinking agent which is apolyester containing two or more beta-hydroxyester groups per molecule.The coating composition can be made cationic and used forelectrodeposition.

It is known in the art that esters containing beta-hydroxyalkyl groupstransesterify very quickly. See, for example, J. PRAKT. CHEM. 312(1970), 660-668. However, European Patent application No. 0012463discloses that polyesters which do not contain beta-hydroxyester groupsbut rather simple ester groups such as methyl esters or butyl esters donot transesterify as readily and are too sluggish to effect sufficientcrosslinking at acceptable conditions.

Surprisingly, it has been found that coating compositions comprisinghydroxyl group-containing polymers and a polyester crosslinking agentwhich do not contain beta-hydroxyester groups can be cured quiteeffectively.

SUMMARY OF THE INVENTION

In accordance with this invention, a coating composition which is heatcurable to give a solvent-resistant coating is provided. The coatingcomposition comprises as the resinous binder:

(A) a polymeric polyol,

(B) a polyester crosslinking agent having at least two beta- and/orgamma-ester ester groups per molecule, and

(C) a transesterification catalyst.

The coating compositions can be made cationic in character such as byusing a polymeric polyol which contains cationic salt groups, theresinous binder dispersed in water and the aqueous dispersion used in amethod of cationic electrodeposition.

DETAILED DESCRIPTION

The polymeric polyol component of the coating compositions can beselected from a wide variety of hydroxyl group-containing polymers suchas alkyd resins, polyester resins, hydroxyl group-containing acrylicpolymers, hydroxyl group-containing epoxy resins and hydroxylgroup-containing resins which are derived from epoxy resins such aspolyepoxide-amine adducts.

The molecular weights of the polymeric polyols can vary over a widerange depending upon their type and on whether the coating compositionis organic solvent based or aqueous based and also on the desiredperformance characteristics of the coating. Polyester, epoxy and alkydresins can have molecular weights as low as about 500 and as high asabout 10,000, preferably the molecular weights are usually in the rangeof about 1,000 to 5,000; the molecular weights being on a weight averagebasis relative to polystyrene, as determined by gel permeationchromatography. Acrylic polymers, on the other hand, can have molecularweights as high as about 100,000, and usually will be in the range ofabout 5,000 to 50,000 on a weight average basis relative to polystyrene,as determined by gel permeation chromatography.

The hydroxyl content of the polymeric polyol should be sufficient suchthat when the polyol is in combination with the curing agent, thecomposition will cure to a solvent-resistant coating. Generally, thehydroxyl number of the polymeric polyol will be at least about 170 andpreferably will be in the range of about 180 to 300, based on resinsolids.

A preferred class of polymeric polyols are hydroxyl group-containingepoxy resins or resins which are derived from epoxy resins such aspolyepoxide-amine adducts which are particularly preferred. The epoxyresins which can be used in the practice of the invention arepolyepoxides, that is, polymers having a 1,2-epoxy equivalency greaterthan 1, preferably about 2 or more. Preferred are polyepoxides which aredifunctional with regard to epoxy. The preferred polyepoxides arepolyglycidyl ethers of cyclic polyols. Particularly preferred arepolyglycidyl ethers of polyphenols such as bisphenol A. Examples ofpolyepoxides are given in U.S. Pat. No. 4,260,716, column 3, line 20, tocolumn 4, line 30, the portions of which are hereby incorporated byreference.

Besides the epoxy resins disclosed above, other epoxy-containingpolymers which can be used are acrylic polymers which contain epoxygroups. These polymers are formed by polymerizing an unsaturated epoxygroup-containing monomer such as glycidyl acrylate or methacrylate withone or more other polymerizable ethylenically unsaturated monomers.Examples of these polymers are described in U.S. Pat. No. 4,001,156,column 3, line 59, to column 5, line 60, the portions of which arehereby incorporated by reference.

Besides the hydroxyl group-containing epoxy resins disclosed above,hydroxyl group-containing polymers derived from epoxy resins such aspolyepoxide-amine adducts can also be used. Examples of amines areammonia, primary, secondary and tertiary amines and mixtures thereof.The reaction product of the polyepoxide and the amine can be at leastpartially neutralized with an acid to form a polymeric productcontaining amine salt and/or quaternary ammonium salt groups. Reactionconditions of polyepoxides with amines, examples of various amines andat least partial neutralization with acid are disclosed in U.S. Pat. No.4,260,720, column 5, line 20, to column 7, line 4, the portions of whichare hereby incorporated by reference.

Also, various polyepoxide-amine adducts are described in European patentapplication No. 0012463.

With regard to the amount of organic amine and polyepoxide which arereacted with one another, the relative amounts depend upon the extent ofcationic salt group formation desired and this in turn will depend uponthe molecular weight of the polymer. The extent of cationic salt groupformation and the molecular weight of the reaction product should beselected such that when the cationic polymer is mixed with aqueousmedium, a stable dispersion will form. A stable dispersion is one whichdoes not settle or is one which is easily dispersible if somesedimentation occurs. In addition, the dispersion should be ofsufficient cationic character that the dispersed resin particles willmigrate towards the cathode when an electrical potential is impressedbetween an anode and a cathode immersed in aqueous dispersion.

Also, the molecular weight, structure and extent of cationic salt groupformation should be controlled such that the dispersed resin will havethe required flow to form a film on the substrate; in the case ofelectrodeposition, to form a film on the cathode. The film should beinsensitive to moisture to the extent that it will not redissolve in theelectrodeposition bath or be rinsed away from the coated surface afterremoval from the bath.

In general, most of the cationic polymers useful in the practice of theinvention will have average molecular weights within the range of about500-100,000 and contain from about 0.01 to 10, preferably about 0.1 to5.0, preferably from about 0.3 to 3.0 milliequivalents of cationic groupper gram of resin solids. Obviously one must use the skill in the art tocouple the molecular weight with the cationic group content to arrive ata satisfactory polymer. The preferrd polyglycidyl ethers will havemolecular weights of about 500 to 10,000, more preferably 1000 to 5,000.Acrylic polymers, on the other hand, will have molecular weights as highas 100,000, preferably 5,000 to 50,000.

Besides epoxy resins and resins derived from epoxy resins, otherhydroxyl group-containing polymers such as alkyd resins, polyesterresins and hydroxyl group-containing acrylic polymers can also be usedin the practice of the invention. Examples of these polymers and theircationic electrodepositable derivatives are shown, for example, inBritish Pat. No. 1,303,480 (hydroxyl group-containing acrylic polymersand polyesters) and British Pat. No. 1,159,390 (hydroxylgroup-containing acrylic polymers).

Besides the cationic polymers which are designed to form aqueous-basedcoating compositions which may be used in coating applications such aselectrodeposition, it should also be appreciated that organicsolvent-based coatings employing the above polymers without cationicsalt groups can also be used. Formulating coating compositions with suchpolymers is well known in the art and need not be described in anyfurther detail.

The crosslinking agent of the coating composition is a polyestercontaining at least two beta- and/or gamma-ester ester groups permolecule. Preferably the polyester is substantially free of polyesterscontaining more than one beta-hydroxyester group per molecule. Bysubstantially free is meant the beta-hydroxyester groups are present inamounts less than that sufficient to achieve a cured coating themselves,i.e., a coating which can withstand 30 acetone double rubs as describedinfra. In general, the beta-hydroxyester groups will be present inamounts less than 5, preferably less than 2 percent by weight calculatedas weight of beta-hydroxyester group per total weight of crosslinker.Usually, the crosslinkers of the invention are completely free ofbeta-hydroxyester groups. Examples of suitable crosslinking agents arethose which are formed from reacting:

(A) a polycarboxylic acid or its functional equivalent with

(B) a member selected from the class of:

(i) 1,2-polyols or 1,2-epoxy compounds,

(ii) 1,3-polyols,

(C) a monocarboxylic acid.

Examples of suitable polycarboxylic acids include dicarboxylic acidssuch as saturated aliphatic dicarboxylic acids, for example, adipic acidand azelaic acid; aromatic acids such as phthalic acid; ethylenicallyunsaturated dicarboxylic acids such as fumaric acid and itaconic acid.

Besides the acids themselves, functional equivalents of the acids suchas anhydrides where they exist and lower alkyl (C₁ -C₄) esters of theacids can be used. Examples include succinic anhydride, phthalicanhydride and maleic anhydride.

Polycarboxylic acids or their functional equivalents having afunctionality greater than two can also be used. Examples includetrimelitic anhydride and polycarboxylic acids formed from reacting adicarboxylic acid with a stoichiometric deficiency of a polyol having afunctionality of 3 or more. For example, the reaction product of adipicacid with trimethylolpropane in a 3:1 molar ratio could be used.

The crosslinking agent can be formed by reacting the polycarboxylic acidor its functional equivalent thereof with the 1,2-polyol, 1,2-epoxycompound, or 1,3-polyol and the monocarboxylic acid in about a 1/2/1equivalent ratio at an elevated temperature, usually reflux temperature,in the presence of an esterification catalyst such as an acid or a tincompound. Usually a solvent, for example, an azeotropic solvent such astoluene or xylene is used. Reaction is continued with water constantlybeing removed until a low acid value, for example, 7 or less, isobtained.

Examples of suitable 1,2-polyols, 1,2-epoxy compounds, and 1,3-polyolsare those of the structure: ##STR1## where R₁, R₂, R₃, R₄, R₅ and R₆ arethe same or different and include hydrogen and the radicals alkyl,cycloalkyl, aryl, alkaryl containing from 1 to 18 carbon atoms,including substituted radicals in which the radicals or the substituentswill not adversely affect the esterification reaction with thepolycarboxylic acid or its functional equivalent thereof, nor adverselyaffect the transesterification curing reaction or the desirableproperties of the coating composition. Examples of suitable substituentsinclude chloro, hydroxy, alkoxy, carboxy, vinyl and when R₁ and R₃ or R₃and R₅ form a closed hydrocarbon ring. Examples of suitable radicalsinclude methyl, ethyl, hydroxymethyl, chloromethyl, carboxymethyl,phenyl, methoxy and phenoxy.

Specific examples of 1,2-polyols include ethylene glycol and propyleneglycol; and examples of 1,2-epoxy compounds include ethylene oxide,propylene oxide, 1,2-epoxy butane, butadiene monoepoxide, glycidol,cyclohexane oxide and a glycidyl ester of a saturated aliphaticmonocarboxylic acid containing from 9 to 12 carbon atoms, i.e., CARDURAE.

Specific examples of 1,3-polyols include 1,3-propanediol,trimethylolpropane, trimethylolethane and 1,3-butanediol.

Examples of monocarboxylic acids are organic monocarboxylic acids havingthe following structure: ##STR2## where R₉ is a hydrocarbamyl radicalcontaining from 1 to 18 carbon atoms. The preferred monocarboxylic acidscontain from 1 to 8 carbon atoms and include acetic, propionic,isobutyric, benzoic and stearic acids.

The crosslinking agent can be formed from reacting the polycarboxylicacid or its functional equivalent thereof with the 1,3-polyol and themonocarboxylic acid at elevated temperature, usually reflux temperature,in the presence of an esterification catalyst such as an acid or a tincompound. Usually a solvent such as toluene or xylene is used. Reactionis continued with water being constantly removed until a low acid value,for example, 7 or less, is obtained.

The third component in the coating compositions of the invention is atransesterification catalyst. These cataysts are known in the art andinclude salts or complexes of metals such as lead, zinc, iron, tin andmanganese. Suitable salts and complexes include 2-ethylhexonates(octoates), naphthanates and acetyl acetonates.

The relative amounts of the polymeric polyol and the crosslinking agentwhich are present in the coating composition can vary between fairlywide limits depending upon the reactivity of the components and the timeand temperature of curing and the properties desired in the curedcoating. In general, the polymeric polyol will be present in amounts ofabout 20 to 95 percent, preferably about 50 to 85 percent by weight, andthe crosslinking agent in amounts of about 5 to 80, preferably 15 to 50percent by weight; the percentages by weight being based on total weightof polymeric polyol and crosslinking agents, and being determined on asolids basis.

The catalyst is present in amounts of about 0.1 to 2.0, preferably about0.2 to 1.0 percent by weight metal based on total weight (solids) of thepolymeric polyol and the crosslinking agent.

The components of the coating composition can be mixed simultaneously orin any order that is convenient. If the components are a liquid and ofsufficiently low viscosity, they can be mixed together neat to form thecoating composition. Alternately, if the components are higher viscosityliquids or solids, the components can be mixed with a diluent to reducethe viscosity of the composition so that it may be suitable for coatingapplications.

By liquid diluent is meant a solvent or a non-solvent which is volatileand which is removed after the coating is applied and is needed toreduce viscosity sufficiently to enable forces available in simplecoating techniques, that is, brushing and spraying, to spread thecoating to controllable, desired, and uniform thickness. Also, diluentsassist in substrate wetting, resinous component compatibility andcoalescence or film formation. Generally, when used, the diluent will bepresent in the composition in amounts of about 20 to 90, preferably 50to 80 percent by weight based on total weight of the coatingcomposition, although more diluent may be employed depending upon theparticular coating application.

Examples of suitable liquid diluents for organic solvent-based coatingswill depend somewhat on the particular system employed. In general,however, aromatic hydrocarbons such as toluene and xylene, ketones suchas methyl ethyl ketone and methyl isobutyl ketone, alcohols such asisopropyl alcohol, normal butyl alcohol, monoalkyl ethers of glycolssuch as 2-alkoxyethanol, 2-alkoxypropanol and compatible mixtures ofthese solvents can be used.

Besides organic solvents, water can be used as a diluent either alone orin combination with water-miscible organic solvents. When water is used,the coating composition is usually modified such as by incorporatingwater-solubilizing groups such as the cationic groups mentioned above toprovide for the necessary solubility in water. Besides the cationicgroups mentioned above, other water-solubilizing groups such asnon-ionic groups, for example, ethylene oxide groups, and anionic groupssuch as carboxylate salt groups may be introduced into the polymericpolyol or the polyester crosslinking agent to disperse or solubilize thecoating composition in water.

The coating compositions of the invention may also optionally contain apigment. Pigments may be of any conventional type, comprising, forexample, iron oxides, lead oxides, strontium chromate, carbon black,coal dust, titanium dioxide, talc, barium sulfate, as well as colorpigments such as cadmium yellow, cadmium red, chromium yellow andmetallic pigments such as aluminum flake.

The pigment content of the coating composition is usually expressed asthe pigment-to-resin weight ratio. In the practice of the presentinvention, pigment-to-resin weight ratios can be as high as 2:1, and formost pigmented coatings, are usually within the range of about 0.05 to1:1.

In addition to the above ingredients, various fillers, plasticizers,anti-oxidants, ultraviolet light absorbers, flow control agents,surfactants and other formulating additives can be employed if desired.These materials are optional and generally constitute up to 30 percentby weight of the coating composition based on total solids.

The coating compositions of the invention can be applied by conventionalmethods including brushing, dipping, flow coating, spraying, and, foraqueous-based compositions containing ionic salt groups, byelectrodeposition. Usually, they can be applied virtually over anysubstrate including wood, metal, glass, cloth, leather, plastic, foamand the like, as well as over various primers. For electroconductivesubstrates such as metals, the coatings can be applied byelectrodeposition. In general, the coating thickness wil vary somewhatdepending upon the application desired. In general, coatings from about0.1 to 10 mils can be applied and coatings from about 0.1 to 5 mils areusual.

When aqueous dispersions of the coating composition are employed for usein electrodeposition, the aqueous dispersion is placed in contact withan electrically conductive anode and an electrically conductive cathode.In the case of cationic electrodeposition, the surface to be coated isthe cathode. Following contact with the aqueous dispersion, an adherentfilm of the coating composition is deposited on the electrode beingcoated when a sufficient voltage is impressed between the electrodes.Conditions under which electrodeposition is carried out are known in theart. The applied voltage may be varied and can be, for example, as lowas 1 volt or as high as several thousand volts, but is typically between50 and 500 volts. Current density is usually between 1.0 ampere and 15amperes per square foot and tends to decrease during electrodepositionindicating the formation of an insulating film.

After the coating has been applied, it is cured by heating at elevatedtemperatures such as at about 150° to 205° C. for about 10 to 45 minutesto form solvent-resistant coatings. By solvent-resistant coatings ismeant that the coating will be resistant to acetone, for example, byrubbing across the coating with an acetone-saturated cloth. Coatingswhich are not cured or poorly cured will not withstand the rubbingaction with acetone and will be removed with less than 10 acetone doublerubs. Cured coatings, on the other hand, will withstand 30 acetonedouble rubs, and preferably at least 100 acetone double rubs.

Illustrating the invention are the following examples which, however,are not to be construed as limiting the invention to their details. Allparts and percentages in the examples, as well as throughout thespecification, are by weight unless otherwise indicated.

EXAMPLE I

The following example shows the preparation of a coating compositioncontaining a crosslinking agent having three beta-ester ester groups permolecule. The crosslinking agent was formed by reacting trimelliticanhydride with ethylene glycol and isobutyric acid in a 1:3:3 molarratio. The crosslinking agent was then mixed with a polymeric polyolformed from condensing an epoxy resin (polyglycidyl ether of apolyphenol) with an amine. The mixture was combined with lead octoatecatalyst and dispersed in water with the aid of acid. Steel panels werecathodically electrocoated with the dispersion and the coatings heatedto give solvent-resistant coatings. Also, the mixture was dissolved inorganic solvent, the solution drawn down on steel panels and the coatedpanels heated to give solvent-resistant coatings. The details of theExample are shown below:

Crosslinking Agent

The crosslinking agent was prepared from the following mixture ofingredients:

    ______________________________________                                                     Weight    Solids    Equiv-                                       Ingredient   (in grams)                                                                              (in grams)                                                                              alents                                                                              Moles                                  ______________________________________                                        Trimellitic anhydride                                                                      192.0     192.0     3.000 1.000                                  Ethylene glycol                                                                            217.2     186.2     7.000 3.500                                  Isobutyric acid                                                                            264.3     264.3     3.000 3.000                                  Para-toluenesulfonic                                                                       1.4       1.4                                                    acid                                                                          Toluene      80.0                                                             ______________________________________                                    

The above ingredients were charged to a reaction vessel under a nitrogenblanket and heated to reflux. Reflux was continued until an acid valueof 4.1 was obtained. The hydroxyl value of the product was 2.9 (apartfrom the acid value), and the water content was 0.03 percent.

Polymeric Polyol

The polymeric polyol was formed from reacting a polyglycidyl ether ofbisphenol A with diethanolamine in about a 3:1 equivalent ratio. Theadduct was then chain extended with a mixture of a primary and adisecondary amine, namely, 3-dimethylaminopropylamine, and the adduct of1,6-hexamethylene diamine and the glycidyl ester of Versatic acid(CARDURA E).

    ______________________________________                                                       Weight  Solids                                                                (in     (in     Equiv-                                         Ingredient     grams)  grams)  alents Moles                                   ______________________________________                                        EPON 829.sup.1 921.0   890.6   4.537  2.268                                                                  >(2.293)                                                                             >(1.147)                                Bisphenol A    255.8   255.8   2.244  1.122                                   Xylene         30.0                                                           Diethanolamine 80.3    80.3    0.765  0.765                                   3-dimethylaminopropyl-                                                                       38.0    38.0    0.745  0.372                                   amine                                                                         1,6-hexamethylene-diamine                                                                    256.8   253.4   0.745  0.372                                   glycidyl ester of Versatic                                                    acid adduct (1:2 molar                                                        ratio).sup.2                                                                  2-butoxyethanol                                                                              614.2                                                          ______________________________________                                         .sup.1 Polyglycidyl ether of bisphenol A having an epoxide equivalent of      about 193-203 commercially available from the Shell Chemical Company.         .sup.2 Adduct formed by adding the glycidyl ester of Versatic acid            dropwise to the 1,6hexamethylene diamine at a temperature of 60° C     At the completion of addition, the mixture was heated to 100° C.       and held for two hours. The glycidyl ester of Versatic acid is                commercially available from Shell Chemical Company as CARDURA E.         

Aqueous Dispersion

An aqueous dispersion of the polymeric polyol and the crosslinking agentprepared as described above was made as follows:

    ______________________________________                                                     Weight     Solids                                                Ingredient   (in grams) (in grams)                                                                              Equivalents                                 ______________________________________                                        Polymeric polyol                                                                           146.2      109.4     0.162 (amine)                               Crosslinking agent                                                                         52.7       40.7                                                  Lead octoate solution                                                                      2.75       2.09.sup.1                                            Lactic acid  7.48                 0.073.sup.2                                 Deionized water                                                                            805.5                                                            ______________________________________                                         .sup.1 75.9% solids lead octoate dissolved in hydrocarbon solvent.            .sup.2 45 percent of the total theoretical neutralization.               

The polymeric polyol, crosslinking agent and lead were charged to astainless steel beaker and mixed together. The lactic acid was added andblended into the mixture, followed by thinning with the deionized water.The aqueous dispersion had a solids content of 14.3 percent.

Untreated and zinc phosphate pretreated steel panels were cathodicallyelectrocoated in the dispersion at 100 volts for 90 seconds. Thecoatings were then cured at 180° C. for 30 minutes to form films havinga thickness of about 0.7 to 0.9 mil. The untreated steel panels had frombetween 56 to 70 acetone double rubs and the zinc phosphate pretreatedsteel panels withstood between 38 to 65 acetone double rubs.

    ______________________________________                                        Organic Solvent-Based Coating Composition                                                      Weight    Solids                                             Ingredient       (in grams)                                                                              (in grams)                                         ______________________________________                                        Polymeric polyol 32.69     27.19                                              Crosslinking agent                                                                             18.12     13.46                                              2-ethoxyethanol  11.82                                                        ______________________________________                                    

The above ingredients were charged to a 2-ounce glass jar and 25.19grams was transferred (after heating and mixing to homogeneity) to asecond 2-ounce glass jar. After transfer, 0.37 gram of lead octoate(75.9 percent solids in a hydrocarbon solvent) was added to form thecoating composition.

The coating composition was drawn down on an untreated steel panel andon a zinc phosphate pretreated steel panel. The coated panels were curedfor 30 minutes at 350° F. (177° C.). The cured coating had a thicknessof about 1.8-2.8 mils and withstood 150 acetone double rubs (untreatedsteel) and 175 acetone double rubs (zinc phosphate pretreated steel).

Coating compositions formulated without lead, applied and cured asdescribed above were removed by only 25 acetone double rubs overuntreated steel and 37 acetone double rubs over zinc phosphatepretreated steel.

EXAMPLE II

The following example shows the preparation of a coating compositioncontaining a crosslinking agent having three gamma-ester ester groupsper molecule. The crosslinking agent was formed by reacting trimelliticanhydride with 1,3-propanediol and isobutyric acid in a 1:3:3 molarratio. The crosslinking agent was mixed with a polymeric polyol asdescribed in Example I, combined with lead octoate catalyst anddispersed in water with the aid of acid. Steel panels were cathodicallyelectrocoated with the dispersion and the coating baked to givesolvent-resistant coatings. Also, the mixture was dissolved in organicsolvent, the solution drawn down on steel panels and the coated panelsheated to give solvent-resistant coatings.

Crosslinking Agent

The crosslinking agent was prepared from the following mixture ofingredients:

    ______________________________________                                                     Weight    Solids    Equiv-                                       Ingredient   (in grams)                                                                              (in grams)                                                                              alents                                                                              Moles                                  ______________________________________                                        Trimellitic anhydride                                                                      80.1      80.1      1.252 0.417                                  1,3-propanediol                                                                            100.0     95.3      2.628 1.314                                  Isobutyric acid                                                                            110.3     110.3     1.252 1.252                                  Para-toluenesulfonic                                                                       1.0        1.0                                                   acid                                                                          Toluene      50.0      --                                                     ______________________________________                                    

The ingredients were charged to a reaction vessel and heated to refluxunder a nitrogen blanket until an acid value of 6.3 was obtained.

Aqueous Dispersion

An aqueous dispersion of the crosslinking agent prepared as describedabove and the polymeric polyol of Example I and lead catalyst wasprepared as follows:

    ______________________________________                                                    Weight     Solids                                                 Ingredient  (in grams) (in grams) Equivalents                                 ______________________________________                                        Polymeric polyol of                                                                       146.2      109.4      0.162 (amine)                               Example I                                                                     Crosslinking agent                                                                        68.1       40.7                                                   Lead octoate                                                                              2.75       2.09                                                   Lactic acid 7.48                  0.073.sup.1                                 Deionized water                                                                           790.1                                                             ______________________________________                                         .sup.1 45 percent of the total theoretical neutralization.               

The polymeric polyol and the crosslinking agent were charged to astainless steel beaker, followed by the addition of the lead octoate.The ingredients were blended together followed by the addition of thelactic acid. The blend was then thinned with deionized water to form a14.4 percent solids dispersion.

Untreated and zinc phosphate pretreated steel panels were cathodicallyelectrocoated in the dispersion at 100 volts for 90 seconds. The coatedpanels were baked for 30 minutes at 180° C. to form coatings having athickness of about 0.9-1.0 mil. The untreated steel panels withstood50-53 acetone double rubs, whereas the coatings on the zinc phosphatedpanels withstood 16-21 acetone double rubs.

    ______________________________________                                        Organic Solvent-Based Coating Composition                                                      Weight    Solids                                             Ingredient       (in grams)                                                                              (in grams)                                         ______________________________________                                        Polymeric polyol 34.43     27.10                                              Crosslinking agent                                                                             22.36     13.35                                              ______________________________________                                    

The above ingredients (20.32 grams) were charged to a 2-ounce glass jarand warmed and mixed to uniformity. The contents were transferred to asecond 2-ounce glass jar and combined with 0.35 grams of lead octoate(75 percent solids) to form the coating composition.

The coating composition was drawn down on steel panels and cured atelevated temperature. The results are presented below:

    ______________________________________                                                  Curing                                                                        Schedule   Coating Thickness                                                                          Acetone                                     Panel     °F./minutes                                                                       (in mils)    Resistance                                  ______________________________________                                        Untreated steel                                                                         350/30     1.9          95-96                                       Zinc phosphate                                                                          350/30     3.2          84                                          pretreated steel                                                              Untreated steel                                                                         400/30     2.0          137-175                                     Zinc phosphate                                                                          400/30     2.8-3.3      >175                                        pretreated steel                                                              ______________________________________                                    

Coating compositions formulated without lead, deposited and cured asdescribed above had the following properties:

    ______________________________________                                                  Curing                                                                        Schedule   Coating Thickness                                                                          Acetone                                     Panel     °F./minutes                                                                       (in mils)    Resistance                                  ______________________________________                                        Untreated steel                                                                         350/30     1.7          30                                          Zinc phosphate                                                                          350/30     2.3          32                                          pretreated steel                                                              Untreated steel                                                                         400/30     2.2          89                                          Zinc phosphate                                                                          400/30     4.0-4.2      91                                          pretreated steel                                                              ______________________________________                                    

EXAMPLE III

The following example shows the preparation of a coating compositioncontaining a crosslinking agent having three gamma-ester ester groupsper molecule as described in Example II. The crosslinking agent wasmixed with a polymeric polyol as described below; the mixture dissolvedin organic solvent and combined with lead octoate catalyst. The coatingcomposition was drawn down on steel panels and heated to givesolvent-resistant coatings.

Polymeric Polyol

The polymeric polyol was formed from chain extending a polyglycidylether of bisphenol A with a polyester diol. The adduct ws then reactedwith a mixture of amines, namely, monoethanolamines and themethylisobutyl diketimine of diethylene triamine.

    ______________________________________                                                       Weight   Solids                                                               (in      (in     Equiv-                                        Ingredients    grams)   grams)  alents Moles                                  ______________________________________                                        EPON 828.sup.1 953.7    953.7   4.819  2.41                                                                   (epoxy)                                       PCP 0200.sup.2 320.6    320.6   1.2    0.6                                                                    (OH)                                          Xylene         80.0                                                           Bisphenol A    274.7    274.7   2.41   1.205                                                                  (OH)                                          Benzyldimethylamine                                                                          5.9                                                            2-ethoxyethanol                                                                              317.9                                                          Methylisobutyl diketimine                                                                    85.7     61.9    0.232  0.232                                  of diethylene triamine.sup.3    (amine)                                       Methylethanolamine                                                                           69.5     69.5    0.926  0.926                                                                  (amine)                                       ______________________________________                                         .sup.1 Polyglycidyl ether of bisphenol A having an epoxide equivalent of      about 198 commercially available from Shell Chemical Company.                 .sup.2 Polycaprolactone diol having a molecular weight of about 545           commercially available from the Union Carbide Company.                        .sup.3 Solution in methylisobutyl ketone.                                

The EPON 828, PCP 0200 and xylene were charged to a reaction vessel andheated under a nitrogen blanket to reflux and held for 30 minutes. Thereaction mixture was cooled to 155° C., followed by the addition of thebisphenol A. Benzyldimethylamine (1.9 grams) was added and the reactionmixture exothermed. The reaction mixture was cooled to 130° C., followedby the addition of the remaining benzyldimethylamine and the reactionmixture held at 130° C. until it attained a reduced Gardner-Holdtviscosity (50 percent resin solids in 2-ethoxyethanol) of N⁺. The2-ethoxyethanol, diketimine and methylethanolamine were then added andthe reaction mixture held at 108°-112° C. for about one hour to completethe reaction.

To 24.23 grams (20.14 grams solids) of the polymeric polyol prepared asdescribed immediately above was added 16.69 grams (9.96 grams solids) ofthe crosslinking agent prepared as described in Example II and 5.6 gramsof 2-ethoxyethanol. To a 20.01 gram sample (12.94 grams solids) of themixture was added 0.23 grams of the lead octoate solution as describedin Example I. The composition was drawn down with a draw bar overuntreated and zinc phosphate pretreated steel panels and the panelsheated to 350° F. (177° C.) for 30 minutes. The cured coatings over theuntreated steel were 1.6 mils in thickness and withstood 57 acetonedouble rubs, whereas the coating over the zinc phosphate pretreatedsteel was 2.8 mils in thickness and withstood 75 acetone double rubs.

When comparable coatings were made without the use of the lead catalyst,the cured coatings withstood only from about 13 to 25 acetone doublerubs.

I claim:
 1. A coating composition which is heat curable to give asolvent-resistant coating comprising:(A) a polymeric polyol, (B) acrosslinking agent having at least two beta- and/or gamma-ester estergroups per molecule, (C) a transesterification catalyst.
 2. Thecomposition of claim 1 in which the crosslinking agent contains at leasttwo beta-ester ester groups per molecule.
 3. The composition of claim 2in which the crosslinking agent is formed from reacting:(A) apolycarboxylic acid or its functional equivalent thereof, (B) a1,2-polyol or 1,2-epoxy compound, (C) a monocarboxylic acid.
 4. Thecomposition of claim 3 in which the 1,2-polyol is selected from theclass consisting of ethylene glycol, propylene glycol, and1,2-butanediol.
 5. The composition of claim 3 in which the 1,2-epoxy isselected from the class consisting of 1,2-epoxy butane, a glycidyl esterof a saturated aliphatic monocarboxylic acid containing from 9 to 12carbon atoms, ethylene oxide, propylene oxide, and n-butyl glycidylether.
 6. The composition of claim 1 in which the crosslinking agentcontains at least two gamma-ester ester groups per molecule.
 7. Thecomposition of claim 6 in which the crosslinking agent is formed fromreacting:(A) a polycarboxylic acid or its functional equivalent thereof,(B) a 1,3-polyol, (C) a monocarboxylic acid.
 8. The composition of claim7 in which the 1,3-polyol is selected from the class consisting of1,3-propanediol, trimethylolpropane, trimethylolethane, and1,3-butanediol.
 9. The composition of claim 3, 4, 5, 7 or 8 in which thepolycarboxylic acid or its functional equivalent thereof is selectedfrom the class consisting of trimellitic anhydride, adipic acid andphthalic anhydride.
 10. The composition of claim 3, 4, 5, 7 or 8 inwhich the monocarboxylic acid is selected from the class consisting ofisobutyric acid, acetic acid, propionic acid and benzoic acid.
 11. Thecomposition of claim 1 in which the polymeric polyol has a hydroxylvalue of at least
 170. 12. The composition of claim 1 in which thepolymeric polyol has a hydroxyl value within the range of 180 to 300.13. The composition of claim 1 in which the polymeric polyol is selectedfrom the class consisting of:(A) a polyglycidyl ether of a cyclicpolyol, (B) the reaction product of a polyglycidyl ether of a cyclicpolyol with an amine, (C) the at least partially acid-neutralizedreaction product of a polyglycidyl ether of a cyclic polyol with anamine.
 14. The composition of claim 13 in which the polyglycidyl etherhas a molecular weight of at least
 500. 15. The composition of claim 1which is dispersed in aqueous medium and in which the polymeric polyolcontains cationic salt groups.
 16. The composition of claim 15 in whichthe cationic salt groups are amine salt groups or quaternary ammoniumbase groups.
 17. The composition of claim 16 which is formed fromreacting:(A) a polyepoxide, (B) an amine; said reaction product being atleast partially neutralized with an acid.
 18. The composition of claim17 in which the polyepoxide is a polyglycidyl ether of a cyclic polyol.